1. Technical Field of the Invention
This invention broadly relates to methods of and additives for treating earthen formations containing clays. The invention further relates to the treatment of subterranean clay-containina formations. The additive of this invention finds particular use in aqueous fluids employed to stimulate the production of fluids. such as hydrocarbons and water, from subterranean clay-containing formations.
2. Description of the Prior Art and Problems Solved
It is well known that earthen formations which contain clays can swell and become unstable if such formations are contacted with waters which are foreign to the formation. It is believed that any salts dissolved in such foreign waters enter into a cation exchange with charged sites existing on the clays and that a common result of this cation exchange is a chanae in the physical structure of the clay, wherein the change is evidenced by the previously mentioned swelling and/or instability.
When clay swells the ability of the clay-containing formation to transmit, that is, to permit the passage of a fluid, such as an aqueous liquid and/or a hydrocarbon liquid, through the formation pores is diminished because the swelling phenomenon reduces the permeability of the material. In certain situations, such as in earthen materials in water ponds and absorbants, wherein liquid transmissibility through the material is not a desired condition, reduced soil permeability is a desired result. However, in situations, such as in subterranean well formations. wherein the ability of a liquid to pass through the material is a desired condition, the obstruction or even an increased resistance to liquid flow through the earthen formation is not a desired result.
Accordingly, for purposes of this invention, the word "damage" and the phrase "formation damage" are employed to describe the effect on an earthen formation, and particularly on one which contains clay, caused by contact of the formation with an aqueous liquid, wherein the effect is manifested by an unwanted increase in the resistance of the formation to permit a liquid to pass through the formation. Clays in such formations are referred to herein as "swelling clays."
In one aspect, formation damage can operate to impede the flow of oil and gas from a subterranean formation into a borehole. As suggested above, clays and fines, when in an undisturbed condition, are stable, accordingly, any obstruction to the flow of hydrocarbons through the stable formation is a function of the natural condition of the formation itself. However, when these materials are placed in contact with an aqueous fluid that is not natural, that is, foreign, to the formation, the clays can swell and the fines can migrate through the capillary flow channels in the formation resulting in a reduction of formation permeability, which has been referred to herein as formation damage. Such damage can be caused by the very fluids employed to enhance, i.e., stimulate, the flow of fluid from the formation. For example, an aqueous based fracturing fluid, employed to make one or more fractures in a hydrocarbon bearing formation to thereby increase the effective permeability of the formation, can, in fact, cause the effective permeability of the formation to decline, if the aqueous base of the fluid causes formation damage. The results of the fracture treatment could, therefore, be unsatisfactory.
Attempts have been made to prevent, or at least to reduce the damaging effects of foreign aqueous fluids upon subterranean formations. Some such attempts have featured dissolving various salts in the aqueous fluids utilized in the treatment of subterranean formations whereby clays contained in the formations, upon contact with the dissolved salts, are converted from a swelling form to a less swelling form. In this regard inorganic salts, such as potassium chloride, sodium chloride, and ammonium chloride, have been dissolved in an aqueous fluid utilized to treat a formation to produce the mentioned conversion. It is believed that the dissolved salts enter into a cation exchange with the charged clay particles to thereby convert the clays from a swelling form to a nonswelling form. While these salts can, and often do, prevent, or at least reduce. damage caused by foreign water on a formation, they can interfere with the desired performance of other constituents of the treatment fluid. For example, salts, which are typically dissolved in an aqueous fluid before admixture with any viscosifyina or gelling agents, can cause a reduction in expected viscosity increase upon the hydration of the gelling agent in the aqueous fluid.
Furthermore, such salts and particularly those containing chloride ions are encountering environmental objections and are therefore preferably to be avoided.
In another aspect, road beds comprised of clay-containing earthen materials are often treated with lime, a potential environmental hazard, to prevent swelling in the presence of water to thereby stabilize the clays to help maintain the structural integrity of roads constructed on clay base materials.
Accordingly, it would be desirable to provide an environmentally acceptable method and additive whereby damage caused by an aqueous treating fluid to an earthen formation, such as a subterranean formation, can be prevented or reduced, wherein the treating fluid can be prepared simply and can be employed without detrimental effects to fluid constituents.